John Chrysostom

But being hungry, they ate simple food, not for pleasure, but on account of the necessity of nature. The Pharisees however, serving the figure and the shadow, accused the disciples of doing wrong. Wherefore there follows, But the Pharisees said unto him, Behold, why do they on the sabbath day that which is not lawful.

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

The disciples of the Lord plucked the ears of grain, as men accustomed to living no longer under the law. When the Pharisees became indignant at this, Christ silences them by pointing to David, who out of necessity broke the law during the time of the high priest Abiathar. Fleeing from Saul, the prophet David came to this high priest and deceived him, saying that he had been sent by the king on an urgent military matter. There he both ate the showbread and took back the sword of Goliath, which he had once dedicated to God. …

Commentary on Mark ↗

Augustine of Hippo

For it was a precept in Israel, delivered by a written law, that no one should detain a thief found in his fields, unless he tried to take something away with him. For the man, who had touched nothing else but what he had eaten, they were commanded to allow to go away free and unpunished. Wherefore the Jews accused our Lord's disciples, who were plucking the ears of corn, of breaking the sabbath, rather than of theft.

Catena Aurea by Aquinas ↗

Bede

Again, they walk through the corn fields with the Lord, who rejoice in meditating upon His sacred words. They hunger, when they desire to find in them the bread of life; and they hunger on sabbath days, as soon as their minds are in a soothing rest, and they rejoice in freedom from troubled thoughts; they pluck the ears of corn, and by rubbing, cleanse them, till they come to what is fit to eat, when by meditation they take to themselves the witness of the Scriptures, to which they arrive by reading, and discuss them …

Catena Aurea by Aquinas ↗